All hail the powerful smile. The right smile, at the right time, wins friends and calms enemies. The smile held for too long, not long enough, flashed too intensively or two dimly, arouses suspicion, fear or anger.

Far from being a straightforward show of joy, the language of smiles is filled with subtlety: a meld of our inner state, surroundings, social training, conscious and unconscious.

The new book Lip Service: Smiles in Life, Death, Trust, Lies, Work, Memory, Sex, and Politics, published August 8, explores the nuances and effects of an expression we use often, but rarely think about. Wired.com spoke with author, Marianne LaFrance, an experimental psychologist at Yale University, about why we smile, how we do it, and the rise of the emoticon.

Wired.com: Are we able to intuitively tell when someone is faking a smile?

Marianne LaFrance: Yes, often, but not always. When experimental studies are done in which fake and apparently spontaneous smiles are shown in pictures or brief videos, to both adults and kids, and the only thing they have to do is mark a smile as genuine or not, people are pretty good at telling the difference.

On the other hand, in more complicated real-world settings most people are not very good.

Wired.com: Why the difference?

LaFrance: In many interactions we’re not paying very close attention. Details that we might catch as not exactly right if we paid attention, we miss because we’re not looking. Also there are a lot of theories about what a real smile looks like. Some of those are patently wrong.

People think they can tell by looking at what the overall face looks like, but in fact there is one muscle [that shows sincerity]. It’s a muscle, called the obicularis occuli, that encircles the eye socket. Most people don’t pay very close attention to and it’s very hard to deliberately adopt. So when people genuinely smile, in a true burst of positive emotion, not only to the corners of the mouth, controlled by the zygomaticus major, but this muscle around the eye also contracts. This causes the crows feet wrinkles that fan out from the outer corners of the eyes and its also responsible for folds in the upper eyelid. Most people can’t do that deliberately.

Wired.com: What about actors and con men?

LaFrance: The muscle doesn’t seem to be under voluntary control normally, but with training and practice, people can learn to use it, some better than others. And of course your garden variety psychopath or Machiavellian personality tends to be better at it. A characteristic of con men is that they somehow manage to exude a positive, good feeling to get themselves into your good graces, only to exploit it.

Wired.com: Are many muscles involved in smiling?

LaFrance: Two, the obicularis occuli and zygomaticus major, are the primary muscles involved in the so-called genuine smile. But the so-called mouth smile can co-occur with a number of other muscles on the face at the same time. So the mouth may be smiling but the brows could be showing anger, the eyes could be showing surprise or fear, the upper lip could be showing contempt, the nose could be showing some disgust. So the smile is interesting to those of us who study it because it’s not just one thing. It’s multiple and complicated.

Even the temporal pattern is important. Genuine smiles tend to come on the face relatively languidly and go off the face in the same kind of way. Fake smiles often seem to snap on the face and snap off.

Wired.com: Humans have always been captivated by smiles. When did smile science get its real start?

LaFrance: In the late 1800s Charles Darwin published his famous book The Expression of Emotions in Man and Animals. He devoted some time to smiling, but was more interested in other facial expressions. His colleague across the channel, Guillaume-Benjamin Duchenne, was doing experiments where he would zap single muscles on the face with electricity and then look at the changes. His primary distinction was between smiles that came from the soul, non-deliberate smiles, and the ones put there consciously. That’s why people who study smiles call the spontaneous smile the Duchenne smile.

Left, non-Duchenne or social smile; right, Duchenne or genuine smile. Image: Megan Mangum.

Wired.com: What does a person’s tendency to smile tell us about them?

LaFrance: Some people are inclined to smile more, and that tends to indicate they are a more upbeat personality. But it’s not a perfect correlation. Some people don’t smile often and are quite content. Others smile a lot of the time, but it is out of anxiety.

Most smiling is social. There are a kazillion situations that, as we become socialized, we learn that many situations call for a smile. Kids learn early on — girls learn much faster than boys — that it is good to fake a smile in some circumstances. Imagine you are given a tacky or disappointing gift; it is a mature behavior to smile and thank the person, because that is the socially appropriate thing to do.

When people are by themselves, believing they are not being observed, there isn’t a lot of smiling. We might think that if we’re reading a book by ourselves and we come across a funny passage we’d break into a smile, but it’s actually pretty rare. But if that same passage is being read to us, or other people are in the room, we’re more likely to smile.

Wired.com: Are there differences in the way males and females smile?

LaFrance: On average girls and women smile more. This appears to be a function of two things. Boys are encouraged not to smile very much. Expressivity is taken by some as sign of emotionality, of femininity, something many men, wouldn’t be caught dead being associated with.

Now women, even when they are not feeling much, are strongly encouraged to look and sound as though they are. Spend some time around young teenage girls and you hear a lot of, “Ooh! Oh, that’s wonderful! Love it! That’s fabulous! Fabulous!” Women who are not very expressive are regarded with some suspicion. They seem cold, withholding, depressed. Acquiring the rules of how expressive one should be with their face is a very socialized process.

Wired.com: Are there differences in the way men and women interpret smiles?

LaFrance: Yes, in general women are more accurate than men in detecting what is really going on with someone by looking at their face and listening to their voice. Women are more likely to tell the difference between a felt and a fake smile.

But there’s another big difference between the perception of smiles. When a woman smiles, men tend to see it as flirtatiousness, even when it has nothing to do with flirtatiousness. Whereas when a woman sees a smile on anybody’s face, man or another woman, she is much more likely to make differentiated judgments and see it as a happy, nervous, embarrassed or fake smile.

Wired.com: Smiles have two ways of getting to our faces, a voluntary and an involuntary way. Why is that?

LaFrance: We don’t really know. Some things we do intentionally, other things spontaneously. Some facial expressions can probably be taken at face value. However, we are also great con artists. It makes evolutionary sense to show expressions that we don’t always feel, and not to show expressions that we do feel. If we always showed exactly what we felt then others could potentially see through to our souls and take advantage of us. But we also occasionally reveal our emotions spontaneously and genuinely, that keeps us on our toes.

Wired.com: Do animals smile?

LaFrance: People often ask me if dogs smile and I have to answer, not exactly endearingly, that dogs don’t smile. Now sometimes it looks like the outer corners of their mouth go up, and it looks like a human smile so it’s interpreted as one. But if smiles are meant to be a display of positive emotion, or a gesture of social recognition, then no. Now, non-human primates do show facial expressions, similar to what we call a smile, that are associated with greetings and camaraderie.

Wired.com: Some medical conditions erase a person’s ability to smile. What do they go through when that happens?

LaFrance: It can be just traumatic. Smiling is so central to our feeling of comfort in the world that, when you’re unable to smile, we are made uncomfortable by that. Or, if you’re with someone who adopts an impassive persona and doesn’t smile, it can be really unnerving.

Recently a woman whose face and eyes had been torn off by a 200-pound chimpanzee had a face transplant. Doctors are now reporting that the transplant seems to be taking hold, because she is able to smell and smile. This woman will be blind for the rest of her life, but it’s remarkable that she’s able to smile. It’ll be important for her life.

Wired.com: What is it about unsmiling people that is unnerving?

LaFrance: People convey by their faces that they acknowledge us, that we’re alive, that we matter, that we are not just objects to be dispensed with.

Wired.com: Why can smiles mean such different things in different cultures?

LaFrance: We acquire ways of knowing who is us and who is them. There have been fascinating studies where Australians and Americans were shown a bunch of face shots of other Australians and Americans. Their task was to identify which nationality, Australian or American, the person was. Shown neutral expressions, accuracy was no better than chance. But shown smiles, they were very good at guessing a person’s nationality. Subtle difference in a person’s smile are detectable, even if we can’t describe why.

Now there are also vast cross-cultural differences in the rules for smiling. Who is it OK to smile at, who not? For how long? For example, often when New Englanders go to the South, they wonder why Southerners are smiling all the time. Sometimes they feel everyone is charming. Sometimes the difference is met with dismay.

Rarely do we think, “Isn’t it interesting that another culture has different smiling rules?” We view them as being a different type of person. Now, at home, judgments based on a person’s smiling habits might be warranted. But when you’re talking about cross-cultural boundaries, those judgments can be really off-base.

Wired.com: Did you come across anything surprising while researching this book?

LaFrance: I found that in obituaries people often, more than any other attribute, mentioned their loved one’s smile. Why is it that, after a person is dead, they are described as someone with a smile? It is one more indicator that smiling is a way we connect with other people, in a way that can be easy to underestimate, but in fact I don’t think its importance can be underestimated.

Wired.com: What do you think about emoticons?

LaFrance: I think they are an interesting invention! Communication is wrought with uncertainties and we have a tough time making sense to each other. When one of the channels is missing, such as facial cues or vocal tone, we’re often lead completely astray. I’m a believer in reducing ambiguity.

Images: 1) gwen/Flickr. 2) Megan Mangum. 3) Duchenne’s experiments, facial expressions triggered by electrical stimuli/Wikimedia.org.